1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter referred to as an LCD) and method of driving the same, in particular, to a LCD having a plurality of pixels in which the adjacent or "contiguous" pixels are supplied with different gray voltages for the same gray scale, thereby resulting in a wider viewing angle for the LCD.
2. Description of the Related Art
An LCD includes an LCD cell having two LCD panels with electrodes and signal lines, such as gate lines and data lines, and liquid crystal material between. The LCD has a backlight unit for providing light to the LCD cell, and it has driving circuit boards and other peripheral circuitry.
A thin film transistor (hereinafter referred to as an TFT) LCD, uses TFTs as switching elements, and it has a plurality of pixels each having a TFT with a gate, a source and a drain, a storage capacitor and a liquid crystal capacitor, as well as gate lines and data lines. Each pixel displays an image according to a data voltage applied to the TFT.
The driving circuit boards contain a gate driver, data drivers, voltage producing circuits and control circuits. The gate driver is connected to the gate lines, and it applies the gate voltages through the gate lines to the gates of the TFTs, thereby activating the TFTs according to the signals from the control circuit. The data driver is connected to the data lines and applies the data voltages through the data lines to the sources of the TFTs to make the pixel display an image according to the signals from the control circuit. The gate driver and the data drivers are supplied with voltages from a voltage producing circuit. The drivers and the voltage producing circuit are controlled by the control circuit.
The LCD, particularly in a twisted nematic (hereinafter referred to as TN) mode, has a narrow viewing angle, and the optical transmission rate through the LCD is a function of the viewing angle. This dependence is due to the characteristics of the liquid crystal molecule and it is greater in the upward and downward direction than in the left to right direction, causing the asymmetrical viewing characteristics. In order to achieve a wider viewing angle, several approaches have been suggested.
Kaneko et al. discloses several methods for obtaining wide viewing angle in "Wide-Viewing-Angle Improvements for AMLCDs", SID 93 DIGEST, pp. 265-268. Suggested improvements include film compensated TN cells, pixel divided TN cells, multi-domain TN cells, and TN cells using subpixels.
Among these prior art methods, the TN cells using three subpixels will be described with reference to FIGS. 1A to 1C. FIG. 1A is an equivalent circuit diagram for a pixel with three subpixels; FIG. 1B shows the plan structure of the pixel, and FIG. 1C shows the cross-sectional structure of the pixel.
In FIG. 1C, a gate electrode 2 is formed on a lower transparent substrate 1 and a first insulating layer 4 is deposited thereon. A first transparent electrode 5 is formed on the first insulating layer 4 and connected to a TFT. A second insulating layer 14 and a second transparent electrode 15 are formed on part of the first transparent electrode 5, and a third insulating layer 24 and a third transparent electrode 25 are formed on part of the second transparent electrode 15. A common electrode 10 is formed on an upper transparent substrate 9.
In FIGS. 1A and 1C, it can be seen that a conventional liquid crystal capacitor is composed of a pixel electrode and a common electrode and is divided into three liquid crystal sub-capacitors CLC1, CLC2 and CLC3. The sub-capacitors CLC1, CLC2 and CLC3 have the first, the second and the third transparent electrodes 5, 15 and 25 respectively as one electrode and the common electrode 10 as the other electrode. In FIG. 1C, it can be seen that CLC1, CLC2, and CLC3 have different values of capacitance, because the capacitance of a capacitor is inversely proportional to the distance between the electrodes of the capacitor and is proportional to the area of the electrodes.
In FIG. 1A, control capacitors CS2 and CS3 connected in series to the sub-capacitors CLC2 and CLC3, respectively, act as voltage dividers and provide control voltages to the subpixels. The transparent electrodes 5, 15 and 25 are supplied with the same voltage, but the voltages VLCi applied between the two electrodes of the sub-capacitors CLCi (i=1, 2 and 3) are different from that applied to the other sub-capacitors. That is, the strength of the electric field applied to the liquid crystal in a subpixel is different from that applied to the other subpixels. Therefore, the pixel has domains with three different optical transmission rates, and the resultant transmission rate of a pixel is the average value of the three transmission rates of the three subpixels. Therefore, the viewing characteristics of the LCD are improved.
However, there is a significant disadvantage in the fabrication process for this type of LCD, because it requires additional steps for forming the second and the third transparent electrodes, and the second and the third insulating layers.